Raw water from ground water sources very often contains iron in the form of soluble ferrous compounds. Its presence causes numbers of undesirable effects, such as taste, staining, and pipe encrustation. Because of this, the acceptable limit in drinking water is set at 0.3 mg/l.
Typical water treatment plants, producing potable water from raw water containing ferrous iron, have the following treatment steps: (a) aeration, (b) the mixing of chemicals such as one or more of lime, potassium-permanganate, and chlorine, (c) slow agitation and formation of oxidized iron in floc particles, (d) sedimentation and removal of precipitated floc as "sludge," and (e) filtration of the supernatant water.
The purpose of the steps in the typical plant and how it removes iron, can be briefly described. Aeration drives off gases which, when dissolved, are acidic; for example, carbon dioxide. This tends to raise the pH of the water. In general, the ultimate formation of precipitated floc is enhanced by elevated pH. The addition of lime, in the form of calcium hydroxide, further raises the pH and a coagulative process occurs as the ferrous compound reacts with the calcium compound, oxidizing the ferrous compound into a flox containing ferric hydroxide. The flocculation time allows building of floc particles to sizes which precipitate rapidly. In the typical plant the sedimentation of floc, or sludge, is dewatered and disposed of.
The present invention differs from conventional treatment in that alum is added to the raw water prior to aeration, and a portion of the sludge is recycled into the upstream process. Lime is added and a detention time is provided so that there is sufficient contact between the above steps and the addition of chlorine in the flocculation tank. I have found that using the alum, prior to aeration, and recycling the sludge, drastically reduces the need for chlorine; however, iron oxidation and removal is enhanced. Thus I have discovered a process which takes less chemicals (and cost) and does a more effective job of iron removal than was previously possible.
Horvath U.S. Pat. No. 4,049,545 disclosed a process of treating "waste water," as he defined the term, by using recycled sludge with alkaline and acidic coagulant aids and precipitants. However, Horvath's invention appears directed to wastes having extremely high solids content and "enormous" bacterial populations (col. 1, lines 23-30). For example, Horvath's first example deals with total solids in the range of 700 mg/l; his second example deals with total solids of 365 mg/l. Total dissolved solids in the second example are reduced only from 272 to 268 mg/l; and initial iron content was only 0.1 mg/l. Removal of suspended solids appears to be the major objective in Horvath; in my invention, the narrow purpose is the removal of dissolved ferrous compounds. Aeration is not used in Horvath's method. Horvath's recycling is taught as an economical coagulant aid; my invention teaches its effectiveness in oxidation and removal of iron.
Cywin U.S. Pat. No. 3,617,559 taught the addition of a "neutralizing agent" into dilute acid waste water; either prior to aeration or partly after aeration (col. 4, line 6). This teaches away from my invention since alum tends to decrease the pH rather than raise it in neutralization. His teaching of splitting part of the agent after aeration also contradicts the teaching of my process. Cywin is concerned principally with neutralization; my invention is concerned with iron removal. Cywin indicates recycling of sludge is well known (col. 2, line 44) in order to increase precipitation. My discovery is, that in combination with pre-aeration addition of alum, there is enhanced oxidation and removal of iron in the ferric state.
Emmett U.S. Pat. No. 3,681,238 teaches the use of alum in stabilizing oil field water wastes, but shows its introduction during and after aeration; unlike my discovery. Emmett teaches the use of alum to coagulate for removal prior to filtration; the use of alum in my invention is to enhance oxidation and removal of iron.
Kostenbader U.S. Pat No. 3,738,932 teaches a method of neutralizing acid mine wastes by injecting alkaline compounds and recycling sludge. But his teaching is away from the present invention in that he, for example, attempts to rid an acid waste of aluminum sulfate (for one example), while my invention teaches the introduction of aluminum sulfate and in a manner which acidizes the raw water.